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    Redox Reactions
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    Write the anode reaction, the cathode reaction and the net cell reaction for the following cells. Which electrode will be the positive terminal in each cell?
    1. space space Zn left parenthesis straight s right parenthesis space vertical line space space Zn to the power of 2 plus end exponent space vertical line vertical line space Br to the power of minus comma space space space Br subscript 2 space vertical line space Pt left parenthesis straight s right parenthesis 2. space space space Cr left parenthesis straight s right parenthesis space vertical line space Cr to the power of 3 plus end exponent space vertical line vertical line space straight I to the power of minus comma space space straight I subscript 2 space straight I space Pt left parenthesis straight s right parenthesis 3. space space space Pt left parenthesis straight s right parenthesis space vertical line space straight H subscript 2 left parenthesis straight g right parenthesis comma space straight H to the power of plus left parenthesis aq right parenthesis space vertical line vertical line space Cu to the power of 2 plus end exponent vertical line space Cu left parenthesis straight s right parenthesis.

    1.   For the cell
        
    Positive terminal:   The bromine electrode where reduction takes place.

    2. For the cell 
    The two half reactions can be represented as:
    (i) Anode reaction (oxidation half reaction):
                    
    (ii) Cathode reaction (reduction half reaction):
                   

    Positive terminal. The iodine electrode where reduction takes place.

    3. For cell 
    the two half reactions can be represented as:
    (i) Anode reaction (oxidation half reaction):
                         
    (ii) Cathode reaction (reduction half reaction):
                       
    Net cell reaction:  It is obtained by adding equations (i) and (ii).

    Positive terminal: The copper electrode where reduction takes place.

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    Answer

    What is electrode potential? Name the factors on which it depends.

    The tendency of an electrode to lose or gain electrons when in contact with the solution of its own ions is known as electrode potential. The tendency of an electrode to lose electrons is a direct measure of its tendency to get oxidised. This tendency is known as oxidation potential.



    The tendency of an electrode to gain electron is a direct measure of its tendency to get reduced. This tendency is known as reduction potential.



    It is clear that the oxidation potential is the reverse of reduction potential. For example, if reduction potential of zinc is –0.76V, its oxidation potential is +0.76V.
    In general, for an electrode,
    Oxidation potential =–Reduction potential

    The magnitude of electrode potential depends on upon three factors:
    (i) Nature of electrode
    (ii) Concentration of the metal ion
    (iii) Temperature.

    Since the electrode potentials depend on upon the concentration of the metal ions in solution and temperature, therefore electrode potential are generally measured under standard conditions i.e. 1 molar concentration of metal ions (1 mol L-1) and a temperature of 298K and are called standard electrode potentials and denoted by E°.
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    What do you know about the standard hydrogen electrode? What is its significance ?
    It consists of platinum foil or wire coated with platinum black dipped into a molar (1 M) solution of H+ ions and hydrogen gas is continuously passed through it at 298 K and 1-atmosphere pressure.



    This electrode may serve as anode or cathode depending upon the nature of another electrode to which it is connected.
    S.H.E. acts as an anode i.e. oxidation takes place, the following reaction occurs:

                        

    S.H.E. acts as a cathode i.e. reduction takes place, the following reaction occurs:

                        

    Thus S.H.E. is a reversible electrode.

                        
    The standard electrode potential (E°) of a standard hydrogen electrode is taken to be 0.00 volt. It is usually represented as Pt, H2(g), (1 atm) [H+ (1 M)] Significance. It is used as a reference electrode to measure the relative value of electrode potential of a given electrode.
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    Consider the following reactions which produce electricity in a galvanic cell:
    left parenthesis straight i right parenthesis space 2 Fe to the power of 2 plus end exponent space plus space Cl subscript 2 left parenthesis straight g right parenthesis space space rightwards arrow space space space 2 Fe to the power of 3 plus end exponent space plus space 2 Cl to the power of minus left parenthesis ii right parenthesis space Cd left parenthesis straight s right parenthesis space plus space straight I subscript 2 space space space rightwards arrow space space space space Cd to the power of 2 plus end exponent space plus space 2 straight I to the power of minus left parenthesis iii right parenthesis space 2 Cr left parenthesis straight s right parenthesis space plus space 3 Cu to the power of 2 plus end exponent space rightwards arrow space space 3 Cu left parenthesis straight s right parenthesis space plus space 2 Cr to the power of 3 plus end exponent
    Write the anode and cathode reactions for galvanic cell. Specify the nature of anodes and cathodes. Write the cell in the usual notation.





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    Split the following redox reactions in oxidation and reduction half reactions:
    left parenthesis straight a right parenthesis space 2 straight K left parenthesis straight s right parenthesis space plus space Cl subscript 2 left parenthesis straight g right parenthesis space space rightwards arrow space space space 2 KCl left parenthesis straight s right parenthesis left parenthesis straight b right parenthesis space 2 Al left parenthesis straight s right parenthesis space plus space 3 Cu to the power of 2 plus end exponent left parenthesis aq right parenthesis space space rightwards arrow space space space 2 Al to the power of 3 plus end exponent left parenthesis aq right parenthesis space plus space 3 Cu left parenthesis straight s right parenthesis

    (a) The given reaction
                 
    (i) Oxidation half-reaction.
                                   
    (ii) Reduction half reaction:
                                  

    (b) The given reaction
              
    (i) Oxidation half reaction:
                        
    (ii) Reduction half reaction:
                   

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